Method and apparatus for controlling a switching mode power supply during transition of load conditions to minimize instability

ABSTRACT

A controller for a switched mode power supply (SMPS) is provided. The SMPS is equipped with a transformer having a primary side winding, a secondary winding, and an auxiliary winding. The controller includes a detection circuit for detecting a transition from a first output load condition to a second output load condition of the SMPS and a control circuit coupled to the detection circuit and being configured to output one or more control signals in response to the detected output load transition. Depending on the embodiment, the one or more control signals include a first control signal for turning on a power switch to cause a current flow in a primary winding of the SMPS and/or one or more second control signals for turning off one or more functional circuit blocks in the controller.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to Chinese Patent Application No.200920163199.4 filed Jul. 22, 2009 by inventors Quanqing Wu, et. al.,commonly assigned and incorporated in its entirety by reference hereinfor all purposes.

BACKGROUND OF THE INVENTION

Embodiments of the present invention are directed to power supplycontrol circuits and power supply systems. More particularly,embodiments of the present invention provide methods and circuits forcontrolling a switching mode power supply (SMPS). Merely as an example,the methods and circuits have been applied in controlling an SMPS duringa transition of load conditions. But embodiments of the invention have amuch wider applicability.

Regulated power supplies are indispensable in modern electronics. Forexample, the power supply in a personal computer often needs to receivepower input from various outlets. Desktop and laptop computers oftenhave regulated power supplies on the motherboard to supply power to theCPU, memories, and periphery circuitry. Regulated power supplies arealso used in a wide variety of applications, such as home appliances,automobiles, and portable chargers for mobile electronic devices, etc.

In general, a power supply can be regulated using a linear regulator ora switching mode controller. A linear regulator maintains the desiredoutput voltage by dissipating excess power. In contrast, a switchingmode controller rapidly switches a power transistor on and off with avariable duty cycle or variable frequency and provides an average outputthat is the desired output voltage.

Compared with linear regulators, switching mode power supplies have theadvantages of smaller size, higher efficiency and larger output powercapability. On the other hand, they also have the disadvantages ofgreater noise, especially Electromagnetic Interference at the powertransistor's switching frequency or its harmonics.

Pulse Width Modulation (PWM) and Pulse Frequency Modulation (PFM) aretwo control architectures of switching mode power supplies. In recentyears, green power supplies are emphasized, which require higherconversion efficiency and lower standby power consumption. In a PWMcontrolled switching mode power supply, the system can be forced toenter into burst mode in standby conditions to reduce power consumption.In a PFM controlled switching mode power supply, the switching frequencycan be reduced in light load conditions. PFM-controlled switching modepower supply exhibits simple control topology and small quiescentcurrent. Therefore, it is suitable for low cost small output powerapplications such as battery chargers and adapters.

Even though widely used, conventional SMPS has many limitations. Forexample, during transition of different output load conditions,especially during relatively large load changes, the SMPS may exhibitunstable output voltages, as described in more detail below.

Therefore, there is a need for techniques that can provide moreeffective control of a switching mode power supply.

BRIEF SUMMARY OF THE INVENTION

The present invention provides devices and methods for controlling theoutput voltage of a switching mode power supply. Merely as an example,the methods and circuits have been applied in controlling an SMPS duringa transition of load conditions.

In an embodiment, a controller according to the invention includes adetection circuit for detecting a change in the output load conditionfrom a heavy load to a light load and a control circuit that disablescertain functional blocks of the controller in response to the detectedload change in order to reduce the current drain of the controller. Ithas been observed that by reducing the current drain of the controllerduring the light load condition, instability at the supply voltage ofthe controller can be minimized. In another embodiment, the instabilitycan be reduced by supplying more power to the output.

In another embodiment of the present invention, a controller deviceincludes a comparator having an input for receiving a feedback signaland configured to compare the feedback signal with a reference voltage.The result of the comparison is then delayed in a delay circuit, and thedelayed control signal is then used to turn on and off a high voltagesource in order to increase the charging rate of the power supply of thecontroller device and to compensate for the current drain when the powerswitch remains inactive.

Some embodiments of the present invention provide a controller for aswitched mode power supply (SMPS) equipped with a transformer having aprimary side winding, a secondary winding, and an auxiliary winding. Thecontroller has a detection circuit for detecting a transition from afirst output load condition to a second output load condition of theSMPS, and a control circuit coupled to the detection circuit andconfigured to output one or more control signals in response to thedetected output load transition. The one or more control signalsincludes a first control signal for turning on a power switch to cause acurrent flow in a primary winding of the SMPS and/or one or more secondcontrol signals for turning off one or more functional circuit blocks inthe controller. In a specific embodiment, the control circuit isconfigured to turn on the power switch and to turn off one or morefunctional blocks in response to the detected output load transition.

Another embodiment of the present invention provides a device forcontrolling a switched mode power supply (SMPS) equipped with atransformer having a primary side winding, a secondary winding, and anauxiliary winding. The device includes a detection circuit for detectinga transition from a heavy output load condition to a light output loadcondition of the SMPS and a control circuit for turning on a powerswitch to cause a current flow in the primary side winding upondetection of the transition. In a specific embodiment, the detectioncircuit comprises a comparator for comparing a feedback voltage with areference voltage. In another embodiment, the feedback voltage isgreater than the reference voltage during the transition.

In some embodiments of the present invention, a switching mode powersupply (SMPS) system includes a transformer with a primary windingcoupled to a power switch, a secondary winding for providing a regulatedoutput voltage, and a controller. In an embodiment, the controller has adetection circuit having an input for receiving a feedback signal andconfigured to detect a change in an output load condition, and a controlcircuit coupled to the detection circuit and configured to output one ormore control signals in response to the detected output load transition.The one or more control signals includes a first control signal forturning on a power switch to cause a current flow in a primary windingof the SMPS and/or one or more second control signals for turning offone or more functional circuit blocks in the controller.

The devices and methods according to the present invention can beapplied both to a conventional pulse width modulator or a pulsefrequency modulator. The circuit and method can also be applied to acontinuous current mode or a discontinuous mode of operation. Parts andfunctions of the present invention include a detector circuit and alogic circuit. In an embodiment of the invention, the detector circuitmay preferably be a comparison circuit and the logic circuit maypreferably be an AND gate.

The devices and methods according to the present invention maypreferably be applied to switched mode power supply systems having atransformer that includes a primary winding, a secondary winding, and anauxiliary winding.

These and other features and advantages of embodiments of the presentinvention will be more fully understood and appreciated uponconsideration of the detailed description of the preferredimplementations of the embodiments, in conjunction with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional AC/DC switching mode powerconverter system;

FIG. 2 is a timing diagram showing certain waveforms of the conventionalAC/DC switching mode power converter system of FIG. 1;

FIG. 3 is a simplified functional block diagram of a switching modepower supply including selected circuit blocks of a controller inaccordance with a first embodiment of the present invention;

FIG. 4 is a simplified block diagram illustrating a switching mode powersupply including selected functional blocks of a controller inaccordance with a second embodiment of the present invention;

FIG. 5 is simplified block diagram of a switching mode power supplyincluding selected functional blocks of a controller for in accordancewith a third embodiment of the present invention.

FIG. 6 is a simplified functional block diagram illustrating selectedblocks of a SMPS controller for a switching mode power supply inaccordance with a fourth embodiment of the present invention;

FIG. 7 is a functional block diagram of a controller for a switchingmode power supply in accordance with a fifth embodiment of the presentinvention;

FIG. 8 is a functional block diagram illustrating selected functionalblocks of a controller for a switching mode power supply in accordancewith a sixth embodiment of the present invention; and

FIG. 9 shows voltage waveforms of a switching mode power supplyaccording to embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a functional block diagram of a conventional AC/DC powerconverter system 100. As shown, system 100 includes an electromagneticinterference (EMI) filter 102, a rectifier 104, a bypass capacitor 106that converts the voltage from the AC voltage source 101 to anunregulated DC voltage at node 105. System 100 further includes atransformer 120 having a primary winding 121, a secondary winding 122,and an auxiliary winding 123. Primary winding 121 is coupled to a switch125 that is switched on and off in response to a drive signal OUT 165.Switch 125 produces a pulsating current 108 across primary winding 121that transfers an magnetic energy to secondary winding 122 and auxiliarywinging 123. On the secondary side, a pulsating current 132 flowingthrough a rectifier 131 is stored in a capacitor 135 that convertspulsating current 132 into a DC voltage, which is further filteredthrough an inductor 137 and a capacitor 138 to provide a substantiallyconstant output voltage Vo to an output load 139.

In FIG. 1, auxiliary winding 123 supplies power to a controller 160through a rectifier 127 and a smoothing capacitor 128. Controller 160includes an input supply terminal Vcc that receives the unregulatedpower supply from node 105 through a resistor 110 and a capacitor 112 atstart up. Controller 160 also includes an under-voltage lockout (UVLO)block, a low voltage drop out (LDO) block, a dc bias block, etc. TheUVLO block is configured to detect the level of supply voltage Vcc andthe bias block is configured to generate a second voltage V1 that isused by some other blocks of controller 160. The UVLO and bias blocksare coupled to the supply voltage Vcc. During the startup of controller160, UVLO block monitors the value of Vcc and prevents controller 160from generating the internal voltage V1 if the value of Vcc is less thana startup value that is required to initiate operations.

Controller 160 also includes a comparator 166 that compares a currentsensing signal 167 with a scaled feedback signal 168. Controller 160further includes an oscillator (OSC) block that, together with theoutput of comparator 166, provides the switching output signal OUT 165to power switch 125 via a driver block 174. Current sense signal 167senses a current 108 flowing across power switch 125. A leading edgeblanking (LEB) block interposed between the current sense signal CS andthe input of comparator 166 blanks any current sensing signals that mayhave high peak magnitudes at the start-up phase for reaching comparator166. Comparator 166 compares the voltage 167 generated by current senseresistor 126 of primary winding 121 and the scaled voltage 168 of anoptocoupler transistor. The compared output signal contains errorinformation of the regulated voltage Vo and serves to set the primarycurrent 108 flowing across power switch 125.

A feedback circuit 140 is coupled to the output voltage Vo to produce,together with an optocoupler 155, a feedback signal 170. Feedbackcircuit 140 includes resistors R10 and R11 that together form a voltagedivider to provide an attenuated voltage of Vo to a shunt regulator 152.Shunt regulator 152 further includes a capacitor C8 and a resistor R12that form a feedback loop compensation circuit. Shunt regulator 152together with optocoupler 155 form an isolated feedback circuit tocontrol the primary current 108. A higher current in the optocoupleroutput transistor results in an decrease in voltage signal FB at theinput of controller 160, and thereby reducing the peak value of theprimary current 108 that then effectively lowering the regulated outputvoltage Vo.

Even though power converter system 100 can be used in some applications,it has many limitations. One of the limitations is that it does nothandle sudden load change conditions satisfactorily, as described inmore detail below.

When output load 139 changes from a heavy load to a light loadcondition, the power consumption in the secondary winding is reduceddramatically and causes a voltage surge at output voltage Vo. Thisvoltage surge is fed back to controller 160 as a feedback signal FB 170via optocoupler 155. Comparator 166 then produces an output controlsignal to driver logic 174 that in turn reduces the primary current 108by turning off power switch 125.

As controller 160 turns off power switch 125, auxiliary winding 123stops supplying pulsating current 129 to charge capacitors 112 and 128.Capacitors 112 and 128 are used for providing power input to the VCC pinof controller 160 and are also referred to as the VCC capacitor or theVcc capacitor. Although controller 160 stops switching power switch 125,it still consumes power because its internal function blocks continue todrain current. This current drainage of internal function blocks causesthe voltage supply Vcc from the VCC pin to fall below the cut-offthreshold value of UVLO.

When output load 139 changes from a heavy load to a light loadcondition, output voltage Vo overshoots and saturates shunt regulator152. In an example, shunt regulator 152 is an adjustable precision shuntregulator AZ431 of BCD Semiconductor Manufacturing Limited. Thesaturation of shunt regulator 152 causes the voltage at FB to drop to avery low level that, in turn, will disable driver logic 174 ofcontroller 160. Consequently, regulated output voltage Vo and supplyvoltage Vcc keep decreasing. When output voltage Vo returns back to itsoriginal target voltage level, the feedback voltage at input FB ofcontroller 160 still remains low because of the large value of C8 of thefrequency compensation circuit. Therefore, power switch 125 remainsdeactivated and supply voltage Vcc continues to drop below UVLO.

FIG. 2 is a timing diagram showing waveforms of supply voltage Vcc,output voltage Vo, switching signal OUT 175 and feedback signal FB 170discussed above. Before time t1, controller 160 operates at a heavy loadcondition and regulates the value of the output voltage Vo to a targetvalue within the desired range of values. Current 129 charges capacitor128 through rectifier 127, and Vcc is above a valid operating thresholdof UVLO. Driver logic 174 is operational and generates switching pulsesOUT to turn on and off power transistor 125 that further maintains thevoltage Vcc and output voltage Vo. The LDO block together with the biasgenerator block generate a second operating voltage v1 that is used bythe FB feedback circuit and other internal blocks of controller 160. Att1, output load 139 changes from the heavy load to a light loadcondition. This load transition causes a voltage surge at Vo. Thevoltage surge is fed back to controller 160 via optocoupler 155.

Controller 160 reacts to the surge as a sign that the output voltageneeds to regulate down, and thus disables power switch 125 to reduceprimary current 108. As a result, output voltage Vo and Vcc continue todrop in value. As capacitors 112 and 128 have relatively smaller valuethan capacitors 135 and 138, voltage Vcc may decrease at a faster ratethan that of Vo. At t2, when Vcc drops below a first threshold valueVth1, an internal electronic circuit (not shown) charges Vcc slowly backto a threshold value Vth2. As shown in FIG. 1, the Vcc capacitor ischarged through resistor 110 from capacitor 106, thus Vcc will increase.In FIG. 2, Vth1 is a start up threshold voltage of IC and Vth2 is a shutdown threshold voltage. At t3, driver logic 174 is operational again andturns on and off power switch 125 that pumps pulsating current 132 viarectifier 131 to capacitor 135 and filter inductor 137 and capacitor138. The surge of output voltage Vo is again fed back to controller 160through optocoupler 155, and the process repeats.

According to embodiments of the present invention, several remedies areprovided to alleviate the problems described above. For example, inorder to prevent Vcc from dropping below UVLO when the output operationmode changes from heavy load to light load, a large bypass capacitor atVcc may be utilized. However, a large bypass capacitor will increase thesystem startup time and cost. Another alternative solution is to reducethe current drain of controller 160 by detecting a load condition changeat the output and by switching off certain functional blocks of thecontroller to reduce power consumption by the controller. Still anotheralternative include detecting a load condition change at the output andturning on the power switch to provide more power.

The following equations provide relationships between the secondaryoutput power and input current for both the continuous current mode(CCM) and discontinuous current mode (DCM):

$\begin{matrix}{P = {V_{AV}*I_{AV}}} & (1) \\{{I_{P\text{-}{PEAK}} = {\frac{I_{O}}{N_{t}\left( {1 - D_{ON}} \right)} + \frac{V_{IN} \cdot D_{ON}}{f_{s} \cdot L}}},{D_{ON} = {\frac{N_{t}V_{O}}{{N_{t}V_{O}} + V_{IN}}\mspace{14mu}({CCM})}}} & (2) \\{I_{P\text{-}{PEAK}} = {I_{RAMP} = {\sqrt{\frac{2 \cdot P_{O}}{f_{s} \cdot L}} = {\frac{V_{IN} \cdot D_{ON}}{f_{s} \cdot L}\mspace{14mu}({DCM})}}}} & (3) \\{{I_{P\text{-}{AV}} = {\frac{I_{O} \cdot D_{ON}}{N_{t}\left( {1 - D_{ON}} \right)} + \frac{V_{IN} \cdot D_{ON}^{2}}{2 \cdot f_{s} \cdot L}}},{D_{ON} = {\frac{N_{t}V_{O}}{{N_{t}V_{O}} + V_{IN}}\mspace{14mu}({CCM})}}} & (4) \\{I_{P\text{-}{AV}} = {{\frac{I_{P\text{-}{PEAK}}}{2} \cdot D_{ON}} = {{\sqrt{\frac{2P_{O}}{f_{s} \cdot L}} \cdot D_{ON}} = {\frac{V_{IN} \cdot D_{ON}^{2}}{2 \cdot f_{s} \cdot L}\mspace{14mu}({DCM})}}}} & (5) \\{D_{ON} = {{\frac{N_{t}V_{O}}{V_{IN}} \cdot \sqrt{\frac{2 \cdot L \cdot f_{s}}{R_{L}}}}\mspace{14mu}({DCM})}} & (7)\end{matrix}$where V_(AV)=average V_(AC); V_(IN)=1.414 V_(AV); I_(AV)=average I_(AC);T_(p-AV)=average I_(AC); L=inductance of the primary winding;N_(t)=N_(p)/N_(s) (ratio between primary and secondary windings);f_(s)=switching frequency, P_(O)=V_(O)*I_(O) (secondary output power),and R_(L)=the output load.

According to some embodiments of the present invention, the instabilityshown in FIG. 2 can be reduced by increasing the charging rate of Vccand/or reducing the current drain of the controller.

FIG. 3 is a simplified functional block diagram of a switching modepower supply 300 including selected circuit blocks of a controller 360in accordance with a first embodiment of the present invention. Some ofthe circuit blocks are similar to those shown in FIG. 1 and are omittedhere to simplify the drawing. As shown, switching mode power supply 300includes a transformer having a primary winding 321, a secondary winding322, and an auxiliary winding 323. Switching mode power supply 300further includes a controller 360. In one embodiment, controller 360 isimplemented using a very high voltage integrated circuit (VHVIC)technology to allow a direct coupling to the high voltage source such asthe primary winding 321. Controller 360 can derive its internal powersupplies from the high voltage input HV. Controller may also receive apower supply from a secondary winding through rectifier 327 and acapacitor 328. At power-on or start up phase, capacitor 328 is chargedthrough the high voltage source to provide a voltage source tocontroller 360. Once Vcc reaches a voltage threshold value, a UVLO & DCBIAS block 331 enables a LDO & Protection circuit block that providesprotection and internal voltage supplies to controller 360. Controller360 turns on and off a power switch 325 coupled to primary winding 321to deliver energy to secondary winding for maintaining a target voltageoutput Vo (not shown). Hardware implementation at the secondary windingside can be similar to the circuitry shown in FIG. 1 including the shuntregulator 152 and opto-coupler 155. In one embodiment, controller 360includes a comparator 310 that compares a feedback signal 370 with areference voltage V2. Comparator 310 produces a logic signal that may beused to disable or enable some or all functional blocks of controller360. In an embodiment, the output of comparator is coupled with a logicgate 320. For example, if feedback signal 370 is below reference voltageV2, i.e., if the output voltage Vo at the secondary side is higher thana target output voltage range, the output of comparator 310 is logic lowand the output of logic gate 320 is asserted low, thus disables UVLOblock 331, LDO block 332, driver logic block 330. In another embodiment,the output of logic gate 320 also disables switch 340 that couples thehigh voltage of the primary winding to the internal Vcc of thecontroller 360 and switch 350 that couples an internal voltage V1 tofeedback signal 370 via a resistor R1. By disabling certain functionalblocks within controller 360, the drain current of controller will bereduced and the voltage Vcc will drop much slower.

FIG. 4 is a simplified block diagram illustrating a switching mode powersupply 400 including selected functional blocks of a controller 460 inaccordance with a second embodiment of the present invention. It isunderstood that controller 460 may include circuit blocks describedabove in connections with FIGS. 1 and 3. Switching mode power supply 400includes a transformer having a primary winding 421, a secondary winding422, and an auxiliary winding 423. Comparator 410 is coupled with afeedback signal ZCD that is derived of a resistive divider R3/R4 coupledto auxiliary winding 423. Feedback signal ZCD reflects both the changeof the regulated output voltage Vo and input voltage (node 105). ZCD canfurther be scaled by multiplying with an average value 453 of thecurrent sensing voltage CS that reflects the current flowing acrosspower switch 425. The scaled product 454 indicating the output power isthen compared with a reference voltage V2 at comparator 410, whoseoutput is coupled to logic circuit 420 for providing control signals.Dependent on the result of the comparison at comparator 410, thesecontrol signals are used to disable some functions of controller 460 toreduce the current drain and to obtain a slower Vcc discharge. Forexample, one such functions is the LDO & Protection circuit block. Inanother example, the control signals can also be used to control thefeedback path of the FB signal.

FIG. 5 is simplified block diagram of a switching mode power supply 500including selected functional blocks of a controller 560 for inaccordance with a third embodiment of the present invention. It isunderstood that controller 560 may include circuit blocks describedabove in connections with FIGS. 1, 3, and 4. As shown in FIG. 5,switching mode power supply 500 includes a transformer having a primarywinding 521, a secondary winding 522, and an auxiliary winding 523.Comparator 510 is coupled with a feedback signal FB that is thencompared with a reference voltage V2. If the voltage of feedback signalFB is lower than reference voltage V2, comparator will produce apositive signal 512 at its output. Positive signal 512 is coupled to anAND gate 520 whose other input is coupled to a clock signal 540. Clockfrequency 540 is used to enable a driver logic block 530 to turn on andoff a power switch 525. By turning on and off power switch 525, thevoltage supply Vcc of controller 560 will be maintained above a voltagethreshold level above UVLO. In other embodiment, this circuit can beeasily adapted to other applications, e.g., to an accelerated buildup ofVcc for the controller at startup.

Feedback signal FB is further coupled with a first input of a comparator534 via a scale circuit k. Comparator 534 has a second input coupledwith a current sensing resistor 526 via a LEB circuit 533. Currentsensing resistor produces a voltage at the CS input of controller 560.In the normal operating mode, comparator 534 compares the voltage CS(after a blanking period at startup) at the current sensing resistor 526and a scaled feedback voltage kFB to produce an error information. Theerror information is then used to activate driver logic circuit 430 toturn on and off power switch for regulating the output voltage Vo.

FIG. 6 is a simplified functional block diagram illustrating selectedblocks of a controller 660 for a switching mode power supply inaccordance with a fourth embodiment of the present invention. It isunderstood that controller 660 may include functional blocks describedabove, depending on the embodiment. In this embodiment, controller 660includes a comparator 610 that receives a feedback signal FB andcompares the FB signal with a reference voltage V2. The result of thecomparison is delayed in a delay circuit 620 to produce a delayedcontrol signal 625. Controller 660 also includes an input HV forreceiving a high voltage source that may be an unregulated directcurrent voltage tapped at a primary winding (not shown). Controller 600further includes a current source 632 that is controlled by an erroramplifier 630. Error amplifier 630 compares a voltage Vcc with areference voltage V1 and produces an error control signal 634. Errorsignal 634 is then used to control current source 632. A switch 640 isinterposed between voltage Vcc and current source 632 and is turned onand off by the delayed control signal 625. As a consequence, thedecrease of the Vcc when a surge at the output voltage Vo during a lightload condition can be slowed down or even compensated by turning ofswitch 640. In an embodiment, switch can be an MOS transistor, atransmission gate, or a semiconductor circuitry.

FIG. 7 is a simplified functional block diagram of a switching modepower supply 700 including selected circuit blocks of a controller 760for in accordance with a fifth embodiment of the present invention. Itis understood that controller 760 may include functional blocksdescribed above, depending on the embodiment. As shown in FIG. 7,controller 700 includes a comparator 734 that compares a scaled feedbacksignal kFB with a current sensing signal CS. Comparator 734 isconfigured to regulate an output voltage Vo to a target value within adesired range of values by turning on and off a power switch 725.Feedback signal FB is further coupled to a comparator 710 that comparesfeedback signal FB with a reference voltage V2. Based on the result ofthe comparison, a logic gate 720 may disable some functional blocks ofcontroller 700 to reduce the current drain, in order to slow down therate of the Vcc voltage drop and thereby minimizing instability atvoltage Vcc and surges at the regulated output voltage when the outputload changes from a heavy load to a light load. In an exemplaryembodiment, LDO and protection block 732 is disabled to reduce the powerconsumption of controller 760. It is appreciated that other functionblocks may also be disabled to save further power consumption ofcontroller 700 and obtain a slower drop rate of Vcc.

FIG. 8 is a functional block diagram illustrating selected functionalblocks of a controller 860 for a switching mode power supply inaccordance with a sixth embodiment of the present invention. It isunderstood that controller 860 may include functional blocks describedabove, depending on the embodiment. In this embodiment, controller 860includes a comparator 810 that compares a feedback signal with areference voltage V2. Based on the result of the comparison, a gate 820may disable certain functional blocks of controller 860 according to anoscillation frequency 840. In an exemplary embodiment, the feedbackinput can be disabled by turning off a switch 850. In an embodiment,disabling the feedback input prevents a sink current from flowing acrossresistor R1 out of controller 860.

FIG. 9 shows voltage waveforms of a switching mode power supplyaccording to embodiments of the present invention. Before time t1, theswitching mode power supply is operating with a heavy load at theoutput. At t1, the output load condition changes from the heavy load toa light load. This load change causes a voltage surge at Vo that is fedback to the controller via its FB input. The voltage surge is detectedby a voltage surge detector, which, as described in connection withFIGS. 3-8 above, disables certain functional blocks of the controller toreduce the current drain and slow down the drop rate of Vcc. As aresult, the waveforms associated with Vcc, Vo, and the feedback signalFB do not show pronounced instability as in the case of a conventionalcontroller (See FIG. 2). And the supply voltage Vcc remains in a validrange that will not activate the under voltage lockout function of thecontroller.

In view of the achieved improvements provided by the illustrativeexamples disclosed above, it is evident that embodiments of the presentinvention not only provide devices and methods to minimize instabilityof the controller voltage supply and the regulated output voltage, butalso can provide circuits and methods to decrease power dissipation ofthe controller when the output condition changes.

While the present invention is described with specific embodiments, itis evident that many alternatives and variations will be apparent tothose skilled in the art. For example, the disclosed devices and methodsof the present invention may also apply to converters with pulse widthmodulation or pulse frequency modulation, and they may also apply tomany other functional blocks such as over current protection block, overtemperature block, and many other functional blocks that are notdisclosed above.

What is claimed is:
 1. A controller for a switched mode power supply(SMPS) equipped with a transformer having a primary side winding, asecondary winding, and an auxiliary winding, the controller comprising:a first control circuit for providing a substantially constant outputvoltage to an output load, the first control circuit including a firstcomparator; a detection circuit including a second comparator configuredfor comparing a feedback voltage with a reference voltage for detectinga transition from a first output load condition to a second output loadcondition of the SMPS; and a second control circuit coupled to thedetection circuit and being configured to output two or more controlsignals in response to the detected output load transition from thefirst output load condition to the second output load condition, the twoor more control signals including: a first control signal for turning ona power switch to cause a current flow in a primary winding of the SMPS;and one or more second control signals for turning off one or morefunctional circuit blocks in the controller, said one or more functionalblocks including one or more of a UVLO (under voltage lockout) block anda driver logic block.
 2. The controller of claim 1 wherein the secondcontrol circuit is configured to turn on the power switch and to turnoff one or more functional blocks in response to the detected outputload transition.
 3. The controller of claim 1 wherein an output of thecomparator is coupled to an AND gate.
 4. The controller of claim 1wherein an output of the comparator is coupled to a driver that iscoupled to the power switch.
 5. The controller of claim 1 wherein anoutput of the comparator is configured to turn on or off a first switch,the first switch being coupled between a HV terminal and a Vcc terminal,said HV terminal being coupled to a high voltage source or the primaryside winding.
 6. The controller of claim 1 wherein an output of thecomparator is configured to turn on or off a second switch, the secondswitch being coupled to a feedback terminal through a first resistor,the second switch also coupled to one or more of the following circuitblocks: an under-voltage lockout (UVLO) circuit; a DC bias circuit; andan enable terminal of a protection circuit.
 7. The controller of claim 1wherein an output of the comparator is connected directly to one or moreof the following circuit blocks: an under-voltage lockout (UVLO)circuit; a DC bias circuit; and an enable terminal of a protectioncircuit.
 8. The controller of claim 1 further comprising an inputvoltage detection circuit having an input coupled to a sample voltagefrom another winding and an output coupled to a multiplier, themultiplier being coupled to a current sense terminal which samples aprimary side current, an output of the multiplier being coupled to thecomparator.
 9. The controller of claim 1 wherein the transition from thefirst output load condition to the second output load condition isassociated with a voltage surge in a feedback signal.
 10. The controllerof claim 1 wherein the control signal maintains its logic state until aload transient occurs between the first and second output loadconditions.
 11. The controller of claim 1 wherein the one or more thefunctional circuit blocks comprise a dc bias generator block.
 12. Thecontroller of claim 1 wherein the one or more functional circuit blockscomprise a low drop out circuit block.
 13. The controller of claim 1wherein the disabling of the one or more functional circuit blocks isperformed by turning off electronic switches, wherein each switch isassociated with the one or more of the functional circuit blocks.
 14. Adevice for controlling a switched mode power supply (SMPS) equipped witha transformer having a primary side winding, a secondary winding, and anauxiliary winding, the device comprising: a detection circuit fordetecting a transition from a heavy output load condition to a lightoutput load condition of the SMPS, the detection circuit including acomparator configured for comparing a feedback voltage with a referencevoltage for detecting a transition from the heavy output load conditionto the light output load condition of the SMPS; and a control circuitfor turning on a power switch to cause an increased current flow in theprimary side winding upon detection of the transition from the heavyoutput load condition to the light output load condition.
 15. The deviceof claim 14 wherein the feedback voltage is greater than the referencevoltage during the transition.
 16. A switching mode power supply (SMPS)system comprising: a transformer with a primary winding coupled to apower switch; a secondary winding for providing a regulated outputvoltage; and a controller, the controller having: a first controlcircuit for providing a substantially constant output voltage to anoutput load, the first control circuit including a first comparator; adetection circuit having an input for receiving a feedback signal andconfigured to detect a change in an output load condition, the detectioncircuit including a second comparator configured for comparing afeedback voltage with a reference voltage for detecting a transitionfrom a first output load condition to a second output load condition ofthe SMPS; and a second control circuit coupled to the detection circuitand being configured to output one or more control signals in responseto a detected output load transition, the one or more control signalsincluding: one or more second control signals for turning off one ormore functional circuit blocks in the controller, said one or morefunctional blocks including one or more of a UVLO (under voltagelockout) block and a driver logic block.
 17. The switching mode powersupply of claim 16 wherein the detection circuit comprises a comparatorthat compares the feedback signal with a reference voltage.
 18. Theswitching mode power supply of claim 16 wherein an output of thecomparator is configured to turn on or off a second switch, the secondswitch being coupled to a feedback terminal through a first resistor,the second switch also coupled to one or more of the following circuitblocks: an under-voltage lockout (UVLO) circuit; a DC bias circuit; andan enable terminal of a protection circuit.